1. Field of the Invention
The present invention relates to mobile radios and, more particularly, to mobile radios in which antennas are equipped for receiving and transmitting radio waves exemplified as mobile phone terminals.
2. Description of the Background Art
The technology relating to mobile communications which is commonly applied to mobile phones, for example, has recently seen a rapid growth. In such mobile phones, antennas are considered especially important, and in keeping with the mobile terminals getting smaller, the antennas are required to be downsized to fit therein.
With reference to the accompanying drawing, described below is an exemplary mobile radio antenna conventionally equipped in mobile phone terminals.
FIG. 8 schematically shows front and cross-sectional side views of a mobile phone terminal, e.g., mobile radio, which has a conventional mobile radio antenna equipped therein.
In FIG. 8, the conventional mobile phone terminal includes a cabinet 101, a display 102 exemplified by a liquid crystal display, a key section 103 exemplified by a ten-key numeric pad, a battery 104, a built-in antenna 106, and a base plate 105 for electrical connections among those constituents. The built-in antenna 106 is structured by an antenna element 106a of a planar configuration, and two metal leads 106b and 106c. This type of built-in antenna 106 is generally called as a planar inverted F antenna (PIFA). The antenna element 106a is provided with a predetermined voltage from a supply point 107 on the base plate 105 via the metal lead 106b. The antenna element 106a is connected to a ground (GND) level of the base plate 105 via the metal lead 106c. Here, the length of a perpendicular line from the antenna element 106a to the base plate 105, i.e., space therebetween, is defined as an antenna height h0.
This antenna height h0 considerably affects the characteristics of the built-in antenna 106 including resonant frequency and frequency bandwidth, i.e., the larger the height h0, the better the antenna characteristics. Other than the height h0, the deciding factors for the antenna characteristics are the size of the antenna element 106a, the positional relationship between the metal leads 106b and 106c, and the like.
However, the conventional mobile radio antenna structured as above has a problem that the larger height h0 results in the thicker mobile phone terminals, thus failing in downsizing.
In order to reduce the thickness of the mobile phone terminals, on the other hand, there is no other choice but to reduce the antenna height h0. This is because the display 102, a speaker, and other constituents which are placed on the other side of the base plate 105 having the built-in antenna 106 disposed thereon cannot be reduced in thickness for this purpose. With the smaller antenna height h0, however, the capacitive coupling between the antenna element 106a and the base plate 105 is increased. This will result in poor matching, and accordingly lower the antenna characteristics.